LONDON – Sergei Skripal, the former Russian double agent attacked by a military-grade nerve agent a month ago, is no longer in critical condition, his doctors said Friday, joining his daughter in a recovery that could lead to the pair helping investigators solve the mystery of exactly how, where and by whom they were poisoned.

Sergei Skripal, 66, is “responding well to treatment, improving rapidly and is no longer in a critical condition,” said Christine Blanshard, medical director at the Salisbury district hospital.

The ex-spy’s 33-year-old daughter, Yulia Skripal, was also poisoned, but her condition had improved to stable last week.

Yulia Skripal is speaking, issuing statements and allegedly talking to relatives back home in Russia on the phone.

“As Yulia herself says, her strength is growing daily and she can look forward to the day when she is well enough to leave the hospital,” her doctor said.

How can this be? As the Russian government has asked, in its campaign to discredit British assertions that Russia was behind the attack, why didn’t Skripals collapse immediately if they were poisoned by a powerful nerve agent? If they came into contact with the agent at their doorstep, how did they have the ability to first go to a pub and a restaurant before they succumbed?

The Skripals were found March 4 on a park bench, she comatose, he awake but disoriented.

Medical experts attributed their recovery to the readily available treatments and antidotes they likely received at the hospital.

“Poisoning does not necessarily result in death,” said Michelle Carlin, a senior lecturer in forensic and analytical chemistry at Northumbria University. “If treated quickly and promptly enough, it is possible to recover from poisoning of many compounds.”

Toxicologists and medical forensic specialists said nerve agents are most dangerous in the first few days after an attack, but given that the Skripals pulled through that, it wasn’t surprising that their condition was improving.

Malcolm Sperrin, a medical physicist, said that a long-term prognosis for the Skripals remained somewhat uncertain. Referring to Yulia, he said “She might talk and converse, but what we don’t know is if she will go back to where she was before the event, not just about cognition, but there’s other aspects that could be affected, like mobility, problems with mental state, anxiety, depression, there are so many unknowns.”

After the doctor’s statement on Friday that the elder Skripal’s health had improved, the Russian Embassy to the United Kingdom tweeted, “Good news!”